1. Technical Field
The present invention relates to an ND filter and a manufacturing method thereof. The ND (neutral density) filter is used for absorbing the amount of incident light so as to uniformly decrease the amount of transmissive light over an entire visible range.
2. Related Art
Generally, when the brightness of an object is too high for an imaging system such as a camera and a video, although minimizing an aperture stop (minimizing an aperture diameter), an excessive amount of light may enter into a light-sensitive surface. To solve this problem, it is a common practice to attach an ND filter to a part of the imaging system so as to limit the amount of light incident on the light-sensitive surface. In this case, the ND filter has spectral characteristics of simply decreasing the amount of incident light and therefore needs to ensure uniform transmittance over an entire visible wavelength range.
Until now, a plastic film-based ND filter has been used for the imaging system such as a camera and a video in order to uniformly decrease the amount of light over the entire visible range. For example, patent document 1 proposes an ND filter composed of alternate layers of two types or more of Ti metal-oxide films (with extinction coefficient k ranging from 1.0 to 3.0) and dielectric films (Al2O3, SiO2, and MgF2). Patent document 2 discloses an ND filter that is composed of alternate layers of SiO2 and Nb and that is fabricated by the roll coating method using sputtering. In addition, patent documents 3 through 6 disclose methods of forming titanium oxide (TiO2) mainly for photocatalyst and optical uses.
[Patent document 1] Japanese Patent Laid open Publication No. Hei 07-063915
[Patent document 2] Japanese Patent Laid open Publication No. 2002-350610
[Patent document 3] Japanese Patent Laid open Publication No. Hei 08-134638
[Patent document 4] Japanese Patent Laid open Publication No. Hei 11-012720
[Patent document 5] Japanese Patent Laid open Publication No. 2000-192226
[Patent document 6] Japanese Patent Laid open Publication No. 2003-231966
With respect to the ND filter described in patent document 1, a process temperature of 150° C. or higher is used to form the light-absorbing film composed of two types or more of Ti metal oxides. When a resin film is used as the substrate, applying a process temperature over 150° C. may deform the substrate or crack a film due to a heat or film stress. The lower-oxide target is used as a raw material for deposition of an absorption film containing two types or more of Ti metal oxides. That lower-oxide target is costly. The ND filter described in patent document 2 uses the thin film laminated structure made by alternately laying a transparent dielectric film of SiO2 and a light-absorbing film of metal Nb. However, this ND filter does not yet reliably ensure the neutrality currently requested in the market for the overall visible range. Patent documents 3 through 6 describe various techniques to form titanium oxide (TiO2). These techniques aim at stably fabricating a transparent film in the visible range. Even the same titanium oxide is not always applicable for yielding the light-absorbing film.